Fig 1.
Three-dimensional surface renderings of the segmented chambers of all specimens used in this study.
A) Spirula spirula, B) pathological S. spirula (pathological chamber indicated by black arrow), C) Nautilus pompilius D) Allonautilus scrobiculatus E) Arnsbergites sp. F) Amauroceras sp. G) Cadoceras sp. H) Kosmoceras sp. Segmented chambers appear in sequentially different colors; only six chambers of Kosmoceras were segmented. The largest segmented chamber is shown in dorsal/ventral view (top) and lateral view (bottom). The boundaries of the chamber volumes trace the shape of the septa. Images are not to scale.
Table 1.
Specimen Data and Tomographic Scan Meta-Data.
Fig 2.
A) Comparison between the surface area to volume ratio (SAC:VC) of each segmented chamber against chamber number for all specimens. B) SAC:VC against shell diameter at each chamber for A. scrobiculatus, S. spirula, Arnsbergites sp., Amauroceras sp., and Kosmoceras sp. SAC:VC is a parameter that reflects the capacity of the shell to compensate for potential buoyancy changes due to the water storing, organic lining in each chamber (Kroger, 2002). Chamber volume (C) and chamber surface area (D) comparisons between S. spirula and selected ammonoids. A. scrobiculatus and N. pompilius have an overall larger volume and surface area due to the much larger size of the animal, maximum diameter is an order of magnitude larger than S. spirula or Kosmoceras. Comparison between S. spirula and the ammonoids is a comparison between extreme morphologies as S. spirula has a whorl interspace, conservative shell cross-section through ontogeny and simple sutures while ammonoids have overlapping whorls, more complex septa (complexity changes through ontogeny), and variable conch morphology and ornamentation. Hm is the potential hatching point, Pa is the pathological chamber, TC is the terminal countdown.
Fig 3.
Siphuncular surface area to chamber volume ratio (SAS:VC) for the three specimens that preserve the siphuncular tube: A. scrobiculatus, S. spirula, and Amauroceras, plotted against chamber number (A) and shell diameter (B).
The siphuncle transfers liquid and gas into and out of the shell, therefore the surface area of the siphuncle limits the diffusion rates of liquid/gas. The higher the SAS:VC the higher the potential rate of diffusion.
Fig 4.
Chamber surface area to chamber volume ratio (A) and siphuncular surface area to chamber volume ratio (B) plotted against cumulative chamber volume.
While both shell diameter and cumulative volume are proxies for size, volume is a more accurate basis for comparison due to the heteromorphic morphology of the shell of Spirula, possessing a whorl interspace that artificially inflates shell diameter. Regardless both graphs show that ammonoids possess a relatively high surface area to volume ratio in early ontogeny.
Fig 5.
Calculated Vogel number for each specimen used in this study.
Vogel number is calculated as the square root of the surface area of the chamber divided by the cube root of the volume of the chamber. Linearizing these values allow direct comparisons between the two while removing scaling effects due to size. It is important to note that the difference between ammonites and S. spirula in early ontogeny exists even when corrected for size. The high values shown by the early chambers of A. scrobiculatus may be an artifact due to resolution and should be interpreted with care.
Fig 6.
Comparison of the curvature between one chamber of A. scrobiculatus and Kosmoceras sp.
Both chambers have similar volume and the chamber of Kosmoceras sp. was resampled to the same voxel size to make the datasets comparable. Curvature is measured at the vertices of the surface mesh. Overall, Kosmoceras sp. shows a consistently higher curvature over a greater percentage of its available surface area. Both chambers show highest curvature along the suture line.